Equipment for high volume manufacture of group iii-v semiconductor materials

ABSTRACT

The invention relates to methods and apparatus that are optimized for producing Group III-N (nitrogen) compound semiconductor wafers and specifically GaN wafers. Specifically, the methods relate to substantially preventing the formation of unwanted materials on an isolation valve fixture within a chemical vapor deposition (CVD) reactor. The invention provides apparatus and methods for limiting deposition/condensation of GaCl 3  and reaction by-products on an isolation valve that is used in the system and method for forming a monocrystalline Group III-V semiconductor material by reacting an amount of a gaseous Group III precursor as one reactant with an amount of a gaseous Group V component as another reactant in a reaction chamber.

FIELD OF THE INVENTION

The present invention relates to the field of semiconductor processingequipment and methods, and provides, in particular, equipment andmethods for the high volume manufacturing of Group III-V compoundsemiconductor wafers that are suitable for fabrication of optic andelectronic components, for use as substrates for epitaxial deposition,and so forth. In preferred embodiments, the equipment and methods aredirected to producing Group III-nitride semiconductor wafers, andspecifically to producing gallium nitride (GaN) wafers.

BACKGROUND OF THE INVENTION

Group III-V compounds are important and widely used semiconductormaterials. Group III nitrides in particular have wide, direct band gaps,which make them particularly useful for fabricating optic components(particularly, short wavelength LEDs and lasers) and certain electroniccomponents (particularly, high-temperature/high-power transistors).

The Group III nitrides have been known for decades to have particularlyadvantageous semiconductor properties. However, their commercial use hasbeen substantially hindered by the lack of readily available singlecrystal substrates. It is a practical impossibility to grow bulk singlecrystal substrates of the Group III-nitride compounds using traditionalmethods, such as Czochralski, vertical gradient freeze, Bridgeman orfloat zone, that have been used for other semiconductors such as siliconor GaAs. The reason for this is the high binding energy of the Ga—N bondwhich results in decomposition, and not melting of GaN at atmosphericpressure. Very high pressure and temperatures (2500° C. and >4 GPapressure are required to achieve melted GaN. While various high pressuretechniques have been investigated, they are extremely complicated andhave lead to only very small irregular crystals. (A. Denis et al, Mat.Sci. Eng. R50 (2006) 167.)

The lack of a native single crystal substrate greatly increases thedifficulty in making epitaxial Group III-nitride layers with low defectdensities and desirable electrical and optical properties. A furtherdifficulty has been the inability to make p-type GaN with sufficientconductivity for use in practical devices. Although attempts to producesemiconductor grade GaN began at least in the early 1970s, no usableprogress was made until the late 1990's when two breakthroughs weredeveloped. The first was the use of low temperature GaN and AlN bufferlayers which led to acceptable growth of Group III-nitride layers onsapphire. The second was the development of a process to achieveacceptable p-type conductivity. In spite of these technologicaladvances, the defect density in Group III-nitride layers is stillextremely high (1E9-1E11 cm⁻³ for dislocations) and the p-typeconductivity is not as high as in other semiconductors. Despite theselimitations, these advances led to commercial production of III-nitrideepitaxial films suitable for LEDs (see, e.g., Nakamura et al, 2nd ed.2000, The Blue Laser Diode, Springer-Verlag, Berlin).

The high defect density is a result of growth on a non-native substrate.Sapphire is the most widely used substrate, followed by silicon carbide.Differences in the lattice constant, thermal coefficient of expansionand crystal structure between the III-nitride epitaxial layer and thesubstrate lead to a high density of defects, stress and cracking of theIII-nitride films or the substrate. Furthermore, sapphire has a veryhigh resistivity (cannot be made conductive) and has poor thermalconductivity.

SiC substrates can be produced in both conductive and highly resistiveforms, but is much more expensive than sapphire and only available insmaller diameters (typically 50 mm diameter with 150 mm and 200 mm asdemonstrations). This is in contrast to sapphire and native substratesfor other semiconductors such as GaAs and silicon, which are availableat lower cost and in much larger diameters (150 mm diameter forsapphire; 300 mm for GaAs).

While the use of sapphire and SiC are suitable for some deviceapplications, the high defect density associated with III-nitride layersgrown on these substrates leads to short lifetime in laser diodes.III-nitride laser diodes are of particular interest because theirshorter wavelength permits much higher information density in opticalrecording methods. It is expected that substrates with lower defectdensities will lead to higher brightness LEDs which are required forreplacement of incandescent and fluorescent bulbs. Finally, GroupIII-nitride materials have desirable properties for high frequency, highpower electronic devices but commercialization of these devices has notoccurred, in part because of substrate limitations. The high defectdensity leads to poor performance and reliability issues in electronicdevices. The low conductivity of sapphire makes it unsuitable for usewith high power devices where it is vital to be able to remove heat fromthe active device region. The small diameter and high cost of SiCsubstrates are not commercially usable in the electronic device market,where larger device sizes (compared to lasers or LEDs) require lowercost, large area substrates.

A large number of methods have been investigated to further reduce thedefect density in epitaxial III-nitrides on non-native substrates.Unfortunately the successful methods are also cumbersome and expensiveand non-ideal even if cost is not an object. One common approach is touse a form of epitaxial lateral overgrowth (ELO). In this technique thesubstrate is partially masked and the III-nitride layer is coerced togrow laterally over the mask. The epitaxial film over the mask has agreatly reduced dislocation density. However, the epitaxial film in theopen regions still has the same high dislocation density as achieved ona non-masked substrate. In addition, further defects are generated whereadjacent laterally overgrown regions meet. To further reduce thedislocation density, one can perform multiple ELO steps. It is clearthat this is a very expensive and time consuming process, and in the endproduces a non-homogeneous substrate, with some areas of low dislocationdensity and some areas with high dislocation density.

The most successful approach to date to reducing defect densities is togrow very thick layers of the III-nitride material. Because thedislocations are not oriented perfectly parallel with the growthdirection, as growth proceeds, some of the dislocations meet andannihilate each other. For this to be effective one needs to grow layerson the order of 300 to 1000 μm. The advantage of this approach is thatthe layer is homogeneous across the substrate. The difficulty is findinga growth chemistry and associated equipment that can practically achievethese layer thicknesses. MOVPE or MBE techniques have growth rates onthe order of less than 1 to about 5 μm/hour and thus are too slow, evenfor many of the ELO techniques discussed above, which require several totens of microns of growth. The only growth technique that hassuccessfully achieved high growth rates is hydride vapor phase epitaxy(HVPE).

In summary, the current state of the art in producing low dislocationGroup III nitride material is to use HVPE to produce very thick layers.However the current HVPE process and equipment technology, while able toachieve high growth rates, has a number of disadvantages. The presentinvention now overcomes these disadvantages and provides relatively lowcost, high quality Group II nitride lead to new, innovativeapplications, e.g., in residential and commercial lighting systems.

SUMMARY OF THE INVENTION

The invention relates to a system for providing high volume productionof a monocrystalline Group III-V semiconductor material. This systemcomprises an external, high volume source of a Group III precursor foruse as one reactant, an external, high volume source of Group Vcomponent for use as another reactant, and a temperature controlledreaction chamber that receives the Group III precursor and Group Vcomponent for reaction to form the monocrystalline Group III-Vsemiconductor material.

In this system, the high volume source of Group III precursoradvantageously comprises a container for holding the precursor, aheating arrangement for heating the precursor and generating a gas flowof the precursor, and a mass flow controller for introducing a desiredamount of gaseous precursor for reaction with the Group V component toform the monocrystalline semiconductor material. Preferably, thecontainer is operatively associated with a source of carrier gas and arelated conduit that introduces the carrier gas into the container in amanner which facilitates formation of the gas flow of the precursor.

In another embodiment, the system further comprises a rotatable supportfor holding one or more substrates upon which the monocrystallinesemiconductor material is to be deposited, and the reaction chamberincludes entry apertures for introducing the reactants into the reactionchamber in a controlled manner. Also, the substrates and support areconfigured, dimensioned and positioned with respect to the reactantentry apertures to facilitate formation and deposition of themonocrystalline semiconductor material upon the substrate(s).

Preferably, the reaction chamber is configured as a horizontal,rectangular chamber and the apertures for introducing the reactants arelocated in different portions of the chamber to avoid reaction untilthey meet at a predetermined location adjacent and immediately above thesubstrate(s). When the reaction chamber includes a floor, a ceiling, apair of sidewalls, an open inlet, and an open outlet, one reactant entryaperture may be a horizontal slot in the floor for introducing thatreactant into the reaction chamber, with the slot configured anddimensioned to introduce the reactant and to direct it to thepredetermined location for reaction with another reactant. Preferably,the reaction chamber has exteriorly cooled walls to prevent or minimizedeposition of reactants thereon.

It is often helpful to heat a reactant prior to introducing it into thereaction chamber. For this purpose, the system may further comprise areactant introduction device that includes a nozzle containing heattransfer materials therein wherein the device is operatively associatedwith a heating source to heat the reactant prior to its introductioninto the reaction chamber. In this embodiment, the entry aperture forthe other reactant may comprise an injection nozzle with the systemfurther including a passage for introducing one or more purge gasstreams into the reaction chamber to direct the other reactant to thepredetermined location to efficiently form the monocrystalline materialon the substrate(s).

Another feature of the system is the provision of a gate valveassociated with the inlet for introducing the purge gas streams in amanner that directs the other reactant to the predetermined location toefficiently form the monocrystalline material upon the substrate(s).When the reaction chamber is made of quartz and the gate valve is madeof metal, an O-ring is typically provided, located between the gatevalve and reactor, for providing a sealing connection. The gate valve iscooled by the purge gas to avoid deteriorating the O-ring duringoperation. The gate valve can also include a plurality of spacedapertures to assist in directing the other reactant to the predeterminedlocation to form the monocrystalline material.

The reaction chamber includes one or more walls and is surrounded by anenclosure which is operatively associated with one or more fans forcirculating air in the enclosure to lower the temperature of thereaction chamber wall(s) to reduce or prevent deposition of the GroupIII precursor or reaction byproducts on the reaction chamber wall(s) toprovide a longer operating time before maintenance is required. Thereaction chamber can also include an outlet and exhaust conduit forremoving exhaust gases, including unreacted Group III precursor,unreacted Group V component and reaction byproducts, from the reactionchamber, and a heating arrangement for heating the outlet and exhaustconduit to a temperature sufficient to prevent condensation of theexhaust gases therein. In addition, the system may further comprise apump for removing the exhaust gases from the reaction chamber, a thermaldestruction chamber for thermally destructing the exhaust gases to formGroup III oxide solids, a scrubber for washing the destructed gases, anda filter for recovering the Group III oxide solids for recycling andformation of additional Group III precursor. In general, the system cansimply include a recovery system for treating unreacted Group IIIprecursor to recover its Group III element for recycling and formationof additional Group III precursor.

Further aspects and details and alternate combinations of the elementsof this invention will be apparent from the appended drawings andfollowing detailed description and these are also within the scope ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention may be understood more fully by reference to thefollowing detailed description of the preferred embodiment of thepresent invention, illustrative examples of specific embodiments of theinvention and the appended figures in which:

FIG. 1 illustrates schematically systems of the invention;

FIGS. 2A-C illustrates preferred GaCl₃ sources;

FIGS. 3A-C illustrates preferred reaction chambers;

FIG. 4 schematically preferred transfer/reaction chamber combinations;

FIG. 5 schematically illustrates preferred inlet manifold structures;and

FIG. 6 illustrates schematically an alternative reactant gas inletarrangement.

The same reference numbers are used to identify the same structuresappearing on different figures.

DETAIL DESCRIPTION OF THE PREFERRED EMBODIMENTS

This invention provides equipment and methods for high growth rate andhigh volume manufacturing of Group III-V compound semiconductor wafersnot hitherto possible. The equipment is capable of sustained productionin that over periods of weeks or months production does not need to beshut down for maintenance. The equipment is capable of high-throughputproduction in that at least a wafer (or a batch of wafers) can beproduced every one to four hours. The Group III-V compound semiconductorwafers so produced are suitable for fabrication of optical andelectronic components, for substrates for further epitaxial depositionand for other semiconductor material applications.

In preferred embodiments, the equipment and methods are specificallydirected to producing GaN nitride wafers, and such embodiments are thefocus of much of the subsequent description. This focus is for brevityonly and should not to be taken as limiting the invention. It will beappreciated that the preferred embodiments can readily be adapted toproducing wafers of other Group III nitrides, e.g., aluminum nitride,indium nitride, and mixed aluminum/gallium/indium nitrides, and toproducing wafers of Group III phosphides and arsenides. Accordingly,producing semiconductors wafers or wafers of any of the III-V compoundsemiconductors are within the scope of this invention.

This invention can be particularly cost effective because particularembodiments can be realized by modifying equipment already commerciallyavailable for epitaxial deposition of Si. Thereby, focus can be on theelements and features that are especially important to GaN epitaxy whileaspects related to high volume manufacturing, which are well developedin silicon technology, can be maintained. Also, the equipment of thisinvention is designed to have a significant duty cycle so that it iscapable of high volume manufacturing. Also, the invention provides forvirtually 100% efficiency in the use of expensive Ga by recovering andrecycling of Ga that is not actually deposited and is thereforeexhausted from the reaction chamber equipment; with limited downtimeneeded. Also, the inventive process and apparatus include an economicaluse of Ga precursors.

The invention includes the use of a known low thermal mass susceptor(substrate holder) and lamp heating with temperature controlled reactorwalls. The use of lamp heating permits the heat energy to mainly becoupled to the susceptor and not heat the reactor walls. The lampheating system is equipped with a control system to permit very fastpower changes to the lamps. The low thermal mass susceptor coupled withthe lamp heating system permit very fast temperature changes, both upand down. Temperature ramp rates are in the range of 2-10 degrees/secondand preferably on the order of 4-7 degrees/second.

The invention includes reactor walls that are controlled to a specifictemperature to minimize undesired gas phase reactions and preventdeposition on the walls. The lack of wall deposition permitsstraightforward use of in-situ monitoring for growth rate, stress andother pertinent growth parameters.

The invention includes one or more external sources for the Group IIIprecursor(s). The flow of the Group III precursor is directly controlledby an electronic mass flow controller. There is no practical limit onthe size of the external Group III source. Group III source containerscan be in the range of 50 to 100 to 300 kg, and several sourcecontainers could be manifolded together to permit switching betweencontainers with no down time. For the deposition of GaN, the Gaprecursor is GaCl₃. This Ga source is based on the observations anddiscoveries that, when GaCl₃ is in a sufficiently low viscosity state,routine physical means, e.g., bubbling a carrier gas through liquidGaCl₃, can provide a sufficient evaporation rate of GaCl₃, and thatGaCl₃ assumes such a sufficiently low viscosity state in a preferredtemperatures of range of 110 to 130° C.

The invention includes equipment for maintaining the GaCl₃ at a constanttemperature and pressure in the low viscosity state and equipment forflowing a controlled amount of gas through the liquid GaCl₃ anddelivering the GaCl₃ vapor to the reactor. This equipment can sustainhigh mass flows of GaCl₃ (in the range of 200 to 400 g/hour) that resultin GaN deposition rates in the range of 100 to 400 μm/hour on one 200 mmdiameter substrates or any number of smaller wafers that fit on thesusceptor. The delivery system from the GaCl₃ container is maintainedwith a specific temperature profile to prevent condensation of theGaCl₃.

The invention also includes an inlet manifold structure that keeps theGroup III and Group V gases separate until the deposition zone and alsoprovides a method for achieving high gas phase homogeneity in thedeposition zone, thus achieving a uniform flow of process gases into thereaction chamber and across the susceptor supporting the substrates. Theprocess gas flow is designed to be substantially uniform in both flowvelocity (therefore, non-turbulent) and chemical composition (therefore,a uniform III/V ratio). In a preferred embodiment, this is realized byproviding separate primary inlet ports for the Group III and Group Vgases that provide uniform distribution of gas across the width of thereactor, and to achieve high uniformity. In preferred embodiments, themanifold and port structures are designed and refined by modeling gasflows according to principles of fluid dynamics.

The invention also includes a method to add energy to either or both theGroup III or Group V inlets to enhance the reaction efficiency of theseprecursors. In a preferred embodiment, this would include a method forthermal decomposition of the dimer form of the Group III precursorGa₂Cl₆ into the monomer GaCl₃. In another preferred embodiment, thiswould include a method for decomposition of the ammonia precursor, forexample by thermal decomposition or plasma.

The invention also includes equipment for automated wafer handling,including fully automatic cassette-to-cassette loading, separate coolingstages, load locks, non-contact wafer handlers, all of which are fullycomputer controlled and interfaced to the overall growth program.

The invention also includes temperature control of the reactor inlet andoutlet flanges and the exhaust system and a specially designed pressureregulating valve that can operate at reduced pressure and hightemperatures. Temperature control in these areas prevents premature gasphase reactions and minimizes deposits of GaN as well as variousreaction byproducts. A major reaction byproduct is NH₄Cl. Thetemperature of the entire exhaust downstream of the reactor iscontrolled to prevent condensation of NH₄Cl.

The invention also includes a gas-purged gate valve to reduce depositson the valve material and the side walls of the reactor and to reducegas recirculation and reduce residence time of the gases in the reactor.

Additional aspects and details of the invention include the use of asusceptor that can hold one or more wafers during one growth run and asusceptor designed to prevent attachment of the substrate to thesusceptor during thick growth runs.

The present invention is based on the discovery that specific metalhalide compounds have certain unique chemical properties, and that whencoupled with an apparatus designed in light of these properties, thecombination can be used to deposit thick layers of Group III-V compoundsemiconductors, and in particular gallium nitride, with heretoforeunachievable high throughput, high uptime and low cost in a mannercharacteristic of high volume manufacturing.

For this invention, “high volume manufacturing” (or HVM) ischaracterized by high throughput, high precursor efficiency and highequipment utilization. Throughput means the number of wafers/hour thatcan be processed. Precursor efficiency means that a large fraction ofthe material input to the system goes into the product and is notwasted. Although there are a large number of variables associated withthe material, process and structure, HVM deposition rates range fromaround 50 g Group III element (such as gallium) per hour for a period ofat least 48 hours, to 100 g Group III element per hour for a period ofat least 100 hours, to 200 g Group III element per hour for a period ofat least one week, to as much as 300 to 400 g Group III element per hourfor a period of at least a month. A typical source capacity can rangefrom 5 Kg to 60 Kg in one vessel and for increased HVM, multiple vesselscan be operated in series. This can provide Group III-V materialthroughputs that are similar to those obtained in silicon manufacture.

Equipment utilization means the ratio of the time that the substrate isin the reactor compared to a given time period, such as 24 hours. ForHVM, most of the time is spent producing product as opposed to set-up,calibration, cleaning or maintenance. Quantitative ranges for thesemeasures are available for mature silicon semiconductor processingtechnology. The equipment utilization for HVM of Group III-V material ison the order of about 75 to 85%, which is similar to that of siliconepitaxial deposition equipment.

Reactor utilization is the period of time during which growth of thematerial on the substrate is occurring in the reactor. For conventionalHVPE reactors, this value is on the order of 40 to 45%, while for a HVMreactor such as those disclosed herein, this value is on the order of 65to 70%.

Growth utilization is the overhead time in the reactor, meaning that itis the time during which growth is occurring in the reactor after asubstrate is provided therein. For conventional HVPE reactors, thisvalue is on the order of 65 to 70%, while for a HVM reactor such asthose disclosed herein, this value is on the order of 95% to close to100%, i.e., close to that of a silicon manufacturing process.

The present invention addresses the main limitations of the current HVPEtechnology which prevent high volume manufacturing. This is done byreplacing the current HVPE in-situ source generation with an externalsource and replacing the current HVPE high thermal mass hot wall reactorwith a low thermal mass reactor with temperature controlled walls. Theuse of an external source eliminates the need to stop production tocharge the precursors, greatly increasing the equipment utilization.Furthermore, the mass flux of the precursor is controlled directly by anelectronic mass flow controller, resulting in improved control of thegrowth process and improve yield. The low thermal mass reactor withtemperature controlled walls greatly reduces the time required forheating and cooling, both during growth and maintenance. The ability torapidly heat and cool the substrate also permits the use ofmulti-temperature processes, which are not practically possible in thecurrent HVPE hot wall system. The ability to control the walltemperature reduces gas phase reactions and almost completely eliminateswall deposits. Elimination of wall deposits greatly increases the timebetween cleaning, leading to high reactor utilization.

The present invention is based on the fact that certain metal halidecompounds can be used as an external source for HVPE deposition of III-Vcompound semiconductors and can provide, in conjunction with specificdelivery equipment detailed in this invention, a sufficiently high massflux to achieve and maintain high deposition rates on large areas. Inparticular, when melted, GaCl₃ is in a sufficiently low viscosity stateto permit routine physical means, e.g. bubbling with a carrier gasthrough liquid GaCl₃, can provide a sufficient evaporation rate ofGaCl₃, and that GaCl₃ assumes such a sufficiently low viscosity state attemperatures in a range about approximately 130° C. Furthermore thisinvention is based on the fact that GaCl₃, in the liquid phase and inthe gas phase at temperatures below about 400° C. is actually a dimer.The chemical formula for the dimer can be written either as (GaCl₃)₂ orGa₂Cl₆.

In addition to Ga₂Cl₆ related chlorogallanes can also be used as a Gaprecursor. These compounds are similar to Ga₂Cl₆ but with H replacingone or more Cl atoms. For example monochlorogallane has the two bridgeCl atoms replace by H atoms. As shown below, the terminal Ga-bondedatoms can also be replaced by H (note that there is a cis and transversion of this compound). According to B. J. Duke et al, Inorg. Chem.30 (1991) 4225, the stability of the dimer decreases with increasingchlorination of the terminal Ga-x bonds by 1-2 kcal/mol per Clsubstitution and increases by 6-8 kcal/mol with each Cl substitution fora bridging H atom. Thus as the number of substituted Cl atoms decreases,the fraction of the monomer, at a given temperature, would decrease.

The growth of In- and Al-containing compounds can be achieved usingsubstantially similar equipment but with the limitation that thesesources are not as easily kept in a liquid state. InCl₃ melts at 583° C.While the present invention described for GaCl₃ may be modified tooperate at temperatures above 583° C., this is practically quitedifficult. An alternate approach is to heat the InCl₃ to a temperaturebelow the melting point but where the vapor pressure is sufficient toachieve acceptable deposition rates.

AlCl₃ sublimes at 178° C. and melts at 190° C. and 2.5 atm. The presentinvention described for GaCl₃ can be modified to operate at higher thanatmospheric pressure and temperatures above the melting point of AlCl₃.Additionally, the alternate approach described above for InCl₃, heatingbelow the melting point to achieve a sufficiently high vapor pressure,will also work. AlCl₃ also forms a dimer (AlCl₃)₂ in the liquid phaseand in the gas phase at low temperatures.

Another main component of this invention is a low thermal mass reactor.The low thermal mass reactor with temperature controlled walls greatlyreduces the time required for heating and cooling, both during growthand maintenance. The ability to rapidly heat and cool the substrate alsopermits the use of multi-temperature processes, which are notpractically possible in the current HVPE hot wall system. The ability tocontrol the wall temperature reduces gas phase reactions and almostcompletely eliminates wall deposits. Elimination of wall depositsgreatly increases the time between cleaning, leading to high reactorutilization.

The low thermal mass is achieved by using what is traditionally called acold wall system, but in this invention the wall temperature iscontrolled to a specific temperature. The current hot wall systems areheated by being enclosed in a furnace. In the new system, only thesubstrate holder and substrate are heated. There are many ways toachieve this including lamp heating, induction heating or resistanceheating. In one embodiment, the system consists of a reactor chamberconstructed from quartz and a substrate heater constructed of graphite.The graphite is heated by lamps on the outside of the quartz reactor.The quartz reactor walls can be controlled using a variety of methods.In most cases the wall temperature control system consists of one ormore methods to measure the wall temperature in a variety of locations,combined with a feedback system to adjust either cooling or heatinginput to the wall region to maintain the temperature at a preset value.In another embodiment, the wall temperature is controlled by fans thatblow air onto the exterior of the reactor walls for cooling. The walltemperature is not constrained to be constant at all times; thetemperature controller can be programmed to vary the temperature toachieve improved performance either during growth or maintenance.

Although the focus of the following description is primarily onpreferred embodiments for producing gallium nitride (GaN) wafers, itwill be appreciated that the equipment and methods described can bereadily adapted by one of average skill in the art to also producewafers of any of the III-V compound semiconductors are within the scopeof this invention. Accordingly, such equipment is within the scope ofthe invention. Headings are used throughout for clarity only and withoutintended limitation.

Also the invention provides equipment for high volume manufacturing ofGaN wafers that is economical to construct and operation. Preferredembodiments of the invention can be economically realized/constructed byadapting/modifying existing VPE equipment that has been designed for andis commercially available for silicon (Si) epitaxy. To practice thisinvention, it is not necessary to undertake an expensive and timeconsuming process of designing and constructing all components for GaNdeposition equipment from scratch. Instead, sustained, high-throughputGaN deposition equipment of the invention can be more rapidly andeconomically realized/constructed making targeted and limitedmodifications to existing Si processing production proven equipment.Along with such modified existing equipment, however, the invention alsoencompasses de novo construction.

Accordingly, the following description is first directed to thegenerally preferred features to be incorporated into existing Siequipment for GaN production. Features that can be retained from Siprocessing are not described in details as they are well known in theart. In different embodiments, different ones of the features to bedescribed can be implemented; the invention is not limited toembodiments implementing all these features. However, for higher levelsof sustained, high-throughout production, most or all of these featuresare advantageous and they include cassette to cassette loading, loadlocks and fully automated wafer handling with separate cooling stagewhich allows fast loading and unloading and processing a wafer while theother one is cooling. The loadlocks eliminate undesirable exposure ofthe reactor to atmosphere to minimize introduction of oxygen and watervapor and greatly reduce purge/bake time before running. Moreover theautomated handling reduces yield loss and wafer breakage from manualhandling of wafers. In some cases a Bernoulli wand is used to handle thewafers which allows hot loading and unloading at temperature as high as900° C. and save long cooling time.

General embodiments of the preferred features of this invention arefirst described in the context of a generic VPE system. It will becomeapparent how these general embodiments can be routinely adapted toparticular, commercially available, Si epitaxy equipment. The followingdescription is then directed to a particular preferred embodiment ofthis invention and of its preferred features that is based on one of theEPSILON® series of single-wafer epitaxial reactors available from ASMAmerica, Inc. (Phoenix, Ariz.). It is apparent, however, that theinvention is not limited to this particular preferred embodiment. Asanother example the inventions could easily be adapted to the CENTURA®series of AMAT (Santa Clara, Calif.).

Preferred embodiments of the equipment and methods of the invention (forproducing GaN wafers) are described in general with reference to FIG. 1.Particular preferred embodiments are then described in more detailedwith reference to FIGS. 2-5. Generally, the equipment of this inventionis designed and sized both for high volume manufacturing of epitaxialGaN layers on substrates and also for economy of construction andoperation.

General Embodiments of the Invention

For convenience and without limitation, the invention is generallydescribed with reference to FIG. 1 in terms of three basic subsystems:subsystems 1 for providing process gases (or liquids); subsystems 3including a reaction chamber; and subsystems 5 for waste abatement.

As noted above, HVM is an attribute of a combination of various physicalfeatures of the system including the generic features described herein:

1. External Source of GaCl3

The structure of the first subsystem, the process gas subsystem,especially the gallium compound vapor source, is an important feature ofthe invention. Known GaN VPE processes are now briefly described. GaNVPE epitaxy comprises synthesizing GaN directly on the surface of aheated substrate from precursor gases containing nitrogen (N) andgallium (Ga) (and, optionally, one or more other Group III metalcontaining gases in order to form mixed nitrides and optionally, one ormore dopants to provide specific electronic conductivity). TheGa-containing gas is usually gallium monochloride (GaCl) or galliumtrichloride (GaCl₃), or a gallium-organic compound, e.g.,tri-ethyl-gallium (TEG) or tri-methyl-gallium (TMG). In the first case,the process is referred to as HVPE (Halide Vapor Pressure Epitaxy and inthe second as MOVPE (Metal Organic Vapor Pressure Epitaxy).

The chemical properties of GaCl (stability only at high temperatures)require that GaCl vapor be synthesized in situ in the reaction vessel,e.g., by passing HCl over a boat containing liquid Ga. In contrast,GaCl₃ is a stable solid at ambient conditions (in the absence ofmoisture) which is commonly supplied in sealed quartz ampoules each withabout 100 g or so. TMG and TEG are volatile liquids. The N-containinggas is usually ammonia (NH₃), and semiconductor quality NH₃ is availablein standard cylinders.

Alternately plasma-activated N₂, e.g., containing N ions or radicals,can be used as the N-containing gas. Molecular N₂ is substantiallyunreactive with GaCl₃ or GaCl even at high process temperatures.Nitrogen radicals can be prepared in a manner known in the art, ingeneral, by providing energy to split a nitrogen molecule, for example,by adding a RF source to a nitrogen line to generate anelectromagnetically induced plasma. When operating in this mode, thepressure in the reactor is usually reduced.

Of the known VPE processes, MOCVD and GaCl HVPE have been found to beless desirable for sustained, high Volume Manufacturing of Group IIInitride layers. First, MOCVD is less desirable for the growth of filmsgreater than 10 um because achievable deposition rates rate are lessthan 5% of the deposition rates achievable by HVPE processes. Forexample, HVPE deposition rate can be in the range of 100-1000 μ/hour ormore, while MOCVD rates are typically less than 10 μ/hour. Second, GaClHVPE is less desirable because this process requires that a supply ofliquid Ga be present in the reaction chamber in order to form GaCl byreaction with HCl. It has been found that maintaining such a supply ofliquid Ga in a form that remains reactive with HCl and that issufficient for high volume manufacturing is difficult.

Therefore, equipment of the invention is primarily directed to GaCl₃HVPE for high volume manufacturing. Optionally, it can also provide forMOCVD for, e.g., deposition of buffer layers and the like. However, useof GaCl₃ HVPE for high volume manufacturing requires a source of GaCl₃vapor that achieves a sufficient flow rate that can be maintainedwithout interruption (except for wafer loading/unloading in the reactionchamber) for a sufficient period. Preferably, an average sustaineddeposition rate is in the range of 100 to 1000 μm/hour of GaN per hourso that approximately one wafer (or one batch of multiple wafers)requires no more than one or two hours of deposition time for even thickGaN layers. Achieving such a preferred deposition rate requires that thesource provide a mass flow of GaCl₃ vapor at about approximately 250 or300 g/hour (a 200 mm circular 300 μm thick layer of GaN comprises aboutapproximately 56 g of Ga while GaCl₃ is about 40% Ga by weight).Further, such a flow rate can preferably be maintained for a sufficientduration so that production interruptions required to recharge/servicethe source are limited to at most one per week, or more preferably oneat least every two to four weeks. Accordingly, it is preferred that theflow rate can be maintained for at least 50 wafers (or batches ofmultiple wafers), and preferably for at least 100, or 150, or 200, or250 to 300 wafers or batches or more. Such a source is not known in theprior art.

The equipment of the invention provides a GaCl₃ source that overcomesproblems in order to achieve preferred flow rates and durations.Achieving preferred flow rates has been hindered in the past by certainphysical properties of GaCl₃. First, at ambient conditions, GaCl₃ is asolid, and vapor can be formed only by sublimation. However, it has beendetermined that GaCl₃ sublimation rates are inadequate for providingvapor at preferred mass flow rates. Second, GaCl₃ melts at about 78° C.,and vapor can then be formed by evaporation from the liquid surface.However, it has also been determined that evaporation rates areinadequate for providing preferred mass flow rates. Further, typicalphysical means for increasing rate of evaporation, e.g., agitation,bubbling, and the like, do not increase evaporation rate sufficientlybecause GaCl₃ liquid is known to be relatively viscous,

What is needed is a form of liquid GaCl₃ of sufficiently lowerviscosity, and it has been observed and discovered that beginning atabout approximately 120° C., and especially at about approximately 130°C. or above, GaCl₃ assumes such a lower viscosity state with a viscositysimilar to, e.g., that of water. And further, it has been observed anddiscovered that in this lower viscosity state, routine physical meansare capable of effectively raising the GaCl₃ evaporation ratesufficiently to provide the preferred mass flow rates.

Accordingly, the GaCl₃ source of this invention maintains a reservoir ofliquid GaCl₃ with temperature T1 controlled to about approximately 130°C. and provides physical means for enhancing the evaporation rate. Suchphysical means can include: agitate the liquid; spray the liquid; flowcarrier gas rapidly over the liquid; bubble carrier gas through theliquid; ultrasonically disperse the liquid; and the like. In particular,it has been discovered that bubbling an inert carrier gas, such as He,N₂ or H₂ or Ar, by arrangements known in the art through a lowerviscosity state of liquid GaCl₃, e.g., GaCl₃ at about 130° C., iscapable of providing the preferred mass flow rates of GaCl₃. Preferredconfigurations of the GaCl₃ source have increased total surface area inproportion to their volume in order to achieve better temperaturecontrol using heating elements outside of the reservoir. For example,the illustrated GaCl₃ source is cylindrical with a height that isconsiderably greater than the diameter. For GaCl₃, this would be around120 g per hour for a period of at least 48 hours, to 250 g per hour fora period of at least 100 hours, to 500 g per hour for a period of atleast one week, to as high as 750 to 1000 g per hour for a period of atleast a month.

Moreover, a GaCl₃ source capable of the preferred flow rate and durationcannot rely on GaCl₃ supplied in individual 100 g ampoules. Such anamount would be sufficient for only 15 to 45 minutes of uninterrupteddeposition. Therefore, a further aspect of the GaCl₃ source of thisinvention is large GaCl₃ capacity. To achieve the high-throughput goalsof this invention, the time spent recharging GaCl₃ source is preferablylimited. However, recharging is made more complicated by the tendency ofGaCl₃ to react readily with atmospheric moisture. The GaCl₃ charge, thesource, and the GaCl₃ supply lines must be free of moisture prior towafer production. Depending on the throughput goals of variousembodiments, the invention includes sources capable of holding at leastabout 25 kg of GaCl₃, or at least about 35 kg, or at least about 50 to70 kg (with an upper limit determined by requirements of size and weightin view of the advantages of positioning the source in close proximityto the reaction chamber). In a preferred embodiment, the GaCl₃ sourcecan hold between about 50 and 100 kg of GaCl₃, preferably between about60 and 70 kg. It will be realized that there is no real upper limit tothe capacity of the GaCl₃ source other than the logistics of itsconstruction and use. Furthermore, multiple sources of GaCl₃ could beset up through a manifold to permit switching from one source to anotherwith no reactor downtime. The empty source could then be removed whilethe reactor is operating and replaced with a new full source.

A further aspect of the GaCl₃ source of this invention is carefultemperature control of the supply lines between the source and thereaction chamber. The temperature of the GaCl₃ supply lines andassociated mass flow sensors, controllers, and the like preferablyincrease gradually from T2 at the exit from the source up to T3 atreaction chamber inlet 33 in order to prevent condensation of the GaCl₃vapor in the supply lines and the like. However, temperatures at thereaction chamber entry must not be so high that they might damagesealing materials (and other materials) used in the supply lines andchamber inlet, e.g., to seal to the quartz reaction chamber, forgaskets, O-rings, and the like. Currently, sealing materials resistantto Cl exposure and available for routine commercial use in thesemiconductor industry generally cannot withstand temperatures greaterthan about 160° C. Therefore, the invention includes sensing thetemperature of the GaCl₃ supply lines and then heating or cooling thelines as necessary (generally, “controlling” the supply linetemperatures) so that the supply line temperatures increase (or at leastdo not decrease) along the supply line from the source, which ispreferably at about approximately 130° C., up to a maximum at thereaction chamber inlet, which is preferably about approximately 145 to155° C. (or other temperature that is safely below the high temperaturetolerance of O-rings or other sealing materials). To better realize thenecessary temperature control, the length of the supply line between thesource apparatus and the reaction chamber inlet should be short,preferably less than about approximately 1 ft., or 2 ft. or 3 ft. Thepressure over the GaCl₃ source is controlled by a pressure controlsystem 17.

A further aspect of the GaCl3 source of this invention is precisecontrol of the GaCl3 flux into the chamber. In a bubbler embodiment, theGaCl3 flux from the source is dependent on the temperature of the GaCl3,the pressure over the GaCl3 and the flow of gas that is bubbled throughthe GaCl3. While the mass flux of GaCl3 can in principle be controlledby any of these parameters, a preferred embodiment is to control themass flux by varying the flow of a carrier gas by controller 21.Routinely-available gas composition sensors such as a Piezocor, and thelike 71 can be used to provide additional control of the actual GaCl3mass flux, e.g., in grams per second, into the reaction chamber. Inaddition, the pressure over the GaCl3 source can be controlled by apressure control system 17 placed on the outlet of the bubbler. Thepressure control system, e.g. a back pressure regulator, allows forcontrol of the over pressure in the source container. Control of thecontainer pressure in conjunction with the controlled temperature of thebubbler and the flow rate of the carrier gas facilitates an improveddetermination of precursor flow rate. Optionally, the container alsoincludes an insulating outer portion.

It is desirable that the materials used in the GaCl3 source, in theGaCl3 supply lines, and in the inlet manifold structures in contact withGaCl3 are chlorine resistant. For metal components, a nickel-based alloysuch as Hastelloy, or tantalum or a tantalum-based alloy is preferred.Further corrosion resistance for metal components can be providedthrough a protective corrosion resistant coating. Such coatings cancomprise silicon carbide, boron nitride, boron carbide, aluminum nitrideand in a preferred embodiment the metal components can be coated with afused silica layer or a bonded amorphous silicon layer, for exampleSILTEK® and SILCOSTEELL® (commercially available from RestekCorporation) has been demonstrated to provide increased corrosionresistance against oxidizing environments. For non-metal components,chlorine resistant polymeric materials (either carbon or siliconepolymers) are preferred.

In view of the above, a preferred GaCl3 source capable of holdingpreferred amounts of GaCl3 is referred to herein as acting“continuously” in that, in an appropriate embodiment, the source candeliver its contained GaCl3 without interruption to deliver the desiredamounts for the recited time durations. It should be understood,however, that, in a particular embodiment, the reaction chamber (orother component of the present system) is or can be so constructed orcertain process details are performed, so that intermittent chambermaintenance, e.g., cleaning and so forth, is required. In contrast, theGaCl3 source is configured and dimensioned to provide the desiredamounts of the precursor in an uninterrupted manner to facilitate highvolume manufacture of the Group III-V product. Thus, the source iscapable of providing these amounts without having to be shut down orotherwise discontinued for replenishment of the solid precursor.

This can be achieved either by providing sufficiently large quantitiesof the solid precursor in a single reservoir, or by providing multiplereservoirs that are manifolded together. Of course, a skilled artisanwould understand that in a manifolded system, one reservoir can beoperated to provide the gaseous precursor while one or more otherreservoirs are being replenished with solid precursor material, and thatthis remains an uninterrupted system since it has no affect on theoperation of the reactor. In such embodiments, the GaCl3 source is alsoreferred to herein as acting continuously in that the source can deliverits contained GaCl3 without refilling, opening, cleaning, replenishingor other procedure during which the source is not fully functional. Inother words, the source does not by itself necessitate interruption ofGaN deposition.

Also, as described, a preferred GaCl3 source can contain GaCl3 in asingle reservoir. Also, a preferred source can include multiplereservoirs (i.e., 2, 5, 10 etc.) having outlets which are manifolded sothat GaCl3 vapor can be delivered from the multiple reservoirs insequence or in parallel. In the following, both embodiments are oftenreferred to as a single source. In preferred embodiments, the equipmentof this invention can also provide for sources for Group III metalorganic compounds so that MOCVD processes can be performed. For example,MOCVD can be used to, e.g., deposit thin GaN or AlN buffer layers, thinintermediate layers, layers of mixed metal nitrides, and so forth.Additional process gases can be routinely supplied as known in the art.

The group V precursor is a gas containing one or more Group V atoms.Examples of such gases include NH₃, AsH₃ and PH₃. For the growth of GaN,NH₃ is typically used because it can provide sufficient incorporation ofN at typical growth temperatures. Ammonia and other N precursors areexternal sources. For example, semiconductor grade NH₃ is readilyavailable in cylinders 19 of various sizes, and carrier gases 72 areavailable as cryogenic liquids or as gases, also in containers ofvarious sizes. Fluxes of these gases can be routinely controlled by massflow controllers 21 and the like. In alternative embodiments, theequipment of this invention can also provide for sources of other GroupIII chlorides.

2. Reactor Geometry

Next, to achieve increased economy, the reactor subsystems arepreferably adaptations of commercially available reactor systems.Available reactors preferred for adaptation and use in this inventioninclude as-is most or all of the features to be next described. Thesefeatures have been determined to be useful for HVM of GaN layers withthe modifications and enhancements disclosed herein. Although thefollowing description is directed mainly to embodiments that adaptexisting equipment, reactors and reactor systems can be purpose built toinclude the to-be-described features. The invention includes bothredesigning and modifying existing equipment and designing andfabricating new equipment. The invention also includes the resultingequipment.

Generally, preferred reaction chambers have horizontal process-gas flowand are shaped in an approximately box-like or hemi-sphere likeconfiguration with lesser vertical dimensions and greater horizontaldimensions. Certain features of horizontal reaction chambers areimportant in limiting unproductive reactor time and achieving HVM ofquality GaN wafers.

3. Low Thermal Mass Susceptor and Lamp Heating

First, time spent ramping-up temperature after introducing new wafersand time spent ramping-down temperature after a deposition run is notproductive and should be limited or minimized. Therefore, preferredreactors and heating equipment also have lower thermal masses (i.e.,ability to absorb heat quickly), and the lower the thermal masses themore preferred. A preferred such reactor is heated with infrared (IR)heating lamps and has IR transparent walls FIG. 1 illustrates reactor 25made of quartz and heated by lower longitudinal IR lamps 27 and uppertransverse IR lamps 29. Quartz is a preferred chamber wall material,since it is sufficiently IR transparent, sufficiently Cl resistant, andsufficiently refractory.

4. Closed Loop Temperature Control on Chamber Walls and Flanges

Time spent cleaning reaction chamber interiors is also not productiveand also should be limited or minimized. During GaN depositionprocesses, precursors, products, or byproducts can deposit or condenseon interior walls. Such deposition or condensation can be significantlylimited or abated by controlling the temperature of the chamber wallsgenerally by cooling them to an intermediate temperature that issufficiently high to prevent condensation of precursors and byproducts,but that is sufficiently low to prevent GaN formation and deposition onthe walls. Precursors used in GaCl3 HVPE processes condense at belowabout 70 to 80° C.; the principal byproduct, NH₄Cl, condenses only belowabout 140° C.; and GaN begins to form and deposit at temperaturesexceeding about 500° C. Chamber walls are controlled to temperature T5that is preferably between 200° C., which has been found to besufficiently high to significantly limit precursor and byproductcondensation, and 500° C., which has been found to be sufficiently lowto significantly limit GaN deposition on chamber walls. A preferredtemperature range for the chamber walls is 250 to 350° C.

Temperature control to preferred ranges generally requires coolingchamber walls. Although IR transparent, chamber walls are neverthelessheated to some degree by heat transferred from the high temperaturesusceptor. FIG. 1 illustrates a preferred cooling arrangement in whichreaction chamber 25 is housed in a full or partial shroud 37 and coolingair is directed through the shroud and over and around the exterior ofthe reaction chamber. Wall temperatures can be measured by infraredpyrometry and cooling air flow can be adjusted accordingly. For example,a multi-speed or a variable speed fan (or fans) can be provided andcontrolled by sensors sensitive to chamber wall temperatures.

5. Load Lock, Cassette to Cassette

Wafer loading and unloading time is also not productive. This time canbe routinely limited by automatic equipment schematically illustrated at39. As it known in the art, this equipment can store wafers, load wafersinto, and unload wafers from the reaction chamber, and generallycomprises, e.g., robotic arms and the like that move wafers, e.g., usingtransfer wands, between external holders and the susceptor in thereaction chamber. During wafer transfer, the reaction chamber can beisolated from ambient exposure by intermediate wafer transfer chambers.For example, controllable doors between the transfer chamber and theexterior can permit loading and unloading and can then seal the transferchamber for ambient exposure. After flushing and preparation, furthercontrollable doors between the transfer chamber and the reactor can opento permit placement and removal of wafers on the susceptor. Such asystem also prevents exposure of the reactor interior to oxygen,moisture or other atmospheric contaminants and reduces purging timesprior to load and unload of wafers. It is preferred to use a quartzBernoulli transfer wand because it reduces unproductive time by allowinghandling of hot wafers without causing contamination.

6. Separate Injection

Process gas flow control, from inlet manifold 33 in the direction ofarrow 31 to outlet manifold 35, is important for depositing high qualityGaN layers. This flow includes the following preferred characteristicsfor the process gases. First, the gallium containing gas, e.g., GaCl₃,and the nitrogen containing gas, e.g., NH₃, preferably enter thereaction chamber through separate inlets. They should not be mixedoutside the reaction chamber because such mixing can lead to undesirablereactions, e.g., forming complexes of GaCl₃ and NH₃ molecules, thatinterfere with subsequent GaN deposition.

Then, after separate entry, the GaCl₃ and NH₃ flows are preferablyarranged so that the gas has a uniform composition in space and timeover the susceptor. It has been found that the III/V ratio should varyover the face of the susceptor (and supported wafer or wafers) at anyparticular time preferably by less than approximately 5%, or morepreferably by less than approximately 3% or 2% or 1%. Also, the III/Vratio should be similarly substantially uniform in time over allportions of the face of the susceptor. Accordingly, the GaCl₃ and NH₃velocity profiles should provide that both gases both spread laterallyacross the width of the reaction chamber so that upon arriving at thesusceptor both gases have a non-turbulent flow that is uniform acrossthe width of the reaction chamber and preferably at least across thediameter of the susceptor.

Finally, the flow should not have recirculation zones or regions ofanomalously low flow rates, where one or more of the process gases canaccumulate with an anomalously high concentration. Localized regions oflow gas flow, or even of gas stagnation, are best avoided.

Preferred process gas flow is achieved by careful design or redesign ofthe inlet manifold of a new or existing reaction chamber. As used herethe term “inlet manifold” refers to the structures that admit processand carrier gases into a reaction chamber whether these structures areunitary or whether they comprise two or more physically separate units.

Inlet manifold designed and fabricated to have the following generalfeatures have been found to achieve preferred process gas flows.However, for most embodiments, it is advantageous for the gas flow intoa selected reaction chamber produced by a proposed inlet manifold designto be modeled using fluid dynamic modeling software packages known inthe art. The proposed design can thereby be iteratively improved toachieve increased uniformity prior to actual fabrication.

First, it has been found advantageous that process gas entry into thereaction chamber be distributed across some, most or all of the width ofthe chamber. For example, multiple gas inlet ports or one or more slotsthrough which gas can enter can be distributed laterally across thewidth of the chamber. A carrier gas such as nitrogen or hydrogen can beintroduced to assist in directing the GaCl₃ and the NH₃ gases throughthe reactor to the desired reaction location above the susceptor.Further, to prevent spurious deposition in the vicinity of the inletports, it is advantageous for the actual inlet ports to be spaced withrespect to the heated susceptor so that they are not heated aboveapproximately 400-500° C. Alternately, the inlet ports can be cooled orcan be spaced apart so the process gases do not mix in their vicinity.

Next it has been found that gas flow properties produced by a particularconfiguration of the GaCl₃ and NH₃ inlets can be improved, or “tuned”dynamically. Secondary purge gas flows impinging on or originating forexample from under the susceptor and mixing with the primary GaCl₃, andNH₃ flows can be used to alter these flows to increase uniformity ofcomposition and velocity or prevent deposition on reactor components.For example, in embodiments where the GaCl₃ and NH₃ flows enter thereaction chamber from different inlets, it has been found advantageousto provide a purge gas flow entering into the reaction chamber somewhatupstream of GaCl₃, and NH₃ flows to confine the process gases above theintended deposition zone and to shield the side walls of the reactorfrom unintended deposition. For these purposes, it is advantageous tointroduce a greater amount of carrier gas laterally near the chamberwalls and a lesser amount centrally about the middle of the chamber.

Also, preferred inlet manifolds provide for dynamic adjustment of, atleast, one of the process gas flows so that non-uniformities observedduring operation can be ameliorated. For example, inlets for a processgas can be divided into two or more streams and individual flow controlvalves can be provided to independently adjust the flow of each stream.In a preferred embodiment, GaCl₃ inlets are arranged into five streamswith independently controllable relative flow.

Further aspects of a preferred inlet manifold include temperaturecontrol. Thereby, inlet manifold temperatures T3 can be controlled bothto prevent the condensation of precursors, e.g., GaCl₃, and to preventdamage to temperature-sensitive materials, e.g., gasket or O-ringmaterials. As discussed, the GaCl₃ inlet ports should be at atemperature no less than the highest temperatures reached in the GaCl₃supply line, which is preferably increased from about approximately 130°C. to about approximately 150° C. Commercially availablechlorine-resistant, sealing materials, such as gasket materials andO-ring materials, available for use in the inlet manifold, in particularfor sealing the manifold to the quartz reaction chamber, begin todeteriorate at temperatures in excess of about approximately 160° C.Chlorine-resistant sealing materials such silicone o-rings usable tohigher temperatures, if available, can also be used, in which case theinlet manifold upper temperature limit can be raised.

Accordingly, inlet manifold temperature T3 should be controlled toremain in the range of about approximately 155 to 160° C. by eithersupplying heat to raise the temperature from ambient or removingtransferred heat from the hot reaction chamber and very hot susceptor.In preferred embodiments, an inlet manifold includes temperature sensorsand channels for temperature control fluids. For example, temperaturesof 155 to 160° C. can be achieved by circulating atemperature-controlled GALDEN™ fluid. Other known fluids can be used forother temperature ranges. The fluid channels preferably run in proximityto the temperature sensitive portions of the inlet manifold, e.g., theGaCl₃ inlet ports and sealing O-rings. Channel arrangement can be chosenmore precisely in view of thermal modeling using software packages knownin the art.

GaCl₃ molecules whether in the solid or liquid or vapor phase are knownto exist mainly in the Ga₂Cl₆ dimer form. That form is actually verystable up to 800° C., Thermodynamic calculations corroborated by gasphase Raman spectroscopy have confirmed that at 300° C. more than 90% ofthe gas phase is composed of the dimer molecule and at 700° C. more than99% of the dimer has decomposed into the GaCl₃ monomer.

As the dimer molecule is injected through a metallic injection port keptat temperatures at or below 150° C., the decomposition of the dimer willoccur only in contact with the hot susceptor which is at temperatureabove 1000° C. Depending on the velocity of the gas above the susceptoror its residence time the portion of the dimer that will be decomposedmight be too small to sustain a high growth rate on the wafer. The GaNdeposition process proceeds through the adsorption of GaCl₃ and itsfurther decomposition to GaCl_(x) with x<3 until all chlorine has beenremoved to obtain an adsorbed atom of Ga. It is therefore desired tooperate from the monomer form of GaCl₃. A preferred embodiment of theinvention introduces the dimer through a quartz tube under the reactorchamber situated upstream of the susceptor region. This quartz tubeconnects to the reactor chamber through a funnel with an ovalcross-section. Energy is provided to the dimer while in the funnel todecompose the dimer to the monomer. A preferred embodiment uses IRradiation from IR lamps located and shaped in such a way that the quartztube and funnel receive a high flux of IR radiation. In this embodiment,the funnel region is filled with IR absorbent materials and theradiation power adjusted to bring the IR absorbent material to atemperature of 600° C. or more preferably 700° C. or higher. As thedimer form of GaCl₃ is injected in the quartz injector and passesthrough the hot funnel zone, the dimer will be decomposed to the monomerand be injected in the reaction chamber just upstream of the susceptor.Preferably the region between the injection point of the GaCl₃ into thereactor and the susceptor is maintained at a temperature above 800° C.to prevent the re-formation of the dimer. A preferred embodiment is touse a SiC plate between the funnel and the susceptor which is heated bythe IR heating lamps to maintain a temperature above 700° C. andpreferably above 800° C.

7. Susceptor and Multi-Wafer Susceptor

The susceptor and its mounting can be of standard construction asgenerally known in the art. For example, it can comprise graphite coatedwith silicon carbide or silicon nitride, or alternatively, a refractorymetal or alloy. The susceptor is preferably mounted for rotation on ashaft. During GaN deposition, susceptor temperatures T4 can beapproximately 1000 to 1100° C. (or higher) and are maintained by thequartz IR lamps controlled by known temperature control circuitry. Toavoid forming a dead zone beneath the susceptor, the susceptor mountingpreferably provides for injection of purge gas. This injection is alsoadvantageous because it can limit or minimize unwanted deposition on theunderside of the heated susceptor and of adjacent components that mayalso be heated (directly or indirectly). The susceptor can be configuredto hold one or more substrates.

8. Heated Exhaust

Reaction chamber outlet manifold 35 provides for the free andunobstructed flow of exhaust gases from the reaction chamber through theexhaust lines 41 and to waste abatement system 5. The exhaust system canalso include a pump (42) and associated pressure control system(pressure control valve (44), pressure gauge (46) and associated controlequipment to permit operation at reduced pressure). The outlet manifoldexhaust lines and pressure control equipment (if used) areadvantageously also temperature controlled to limit condensation ofreaction products. Exhaust gases and reaction products typicallycomprise the carrier gases; un-reacted process gases, GaCl₃ and NH₃;reaction byproducts which are primarily NH₄Cl, NH₄GaCl₄, HCl, and H₂. Asdescribed above, temperatures above about approximately 130° C. arerequired to prevent condensation of GaCl3. NH₄Cl condenses into apowdery material below about approximately 140° C., and the outletmanifold and exhaust system should be kept above this temperature. Onthe other hand, to prevent deterioration of sealing materials, theoutlet manifold temperature should not exceed about approximately 160°C.

Accordingly, outlet manifold temperature T6 is preferably maintained inthe range of about approximately 155 to 160° C. by temperature controlmeans similar to those used for inlet manifold temperature control(including optional thermal modeling). Maximum exhaust line temperatureT7 is limited by the maximum allowable temperature for the seals,preferably in the range of about 155 to 160° C.

9. Waste Management

Considering next waste abatement subsystems 5, a preferred abatementsystem can assist in economical operation of the invention by recoveryof waste gallium compounds exhausted from the reaction chamber. A singleembodiment of the invention can exhaust 30 kg, or 60 kg, or more during(assuming approximately 50% waste) during a month of sustained, highvolume manufacturing. At current Ga prices, it is economical to recoverthis waste Ga and recycle it into GaCl₃ precursor, thereby achievingeffectively approximately 90 to 100% Ga efficiency.

FIG. 1 also schematically illustrates a preferred embodiment of wasteabatement subsystem 5 that provides for gallium recovery and that can bereadily adapted from commercially available products. The streamexhausted from reaction chamber 25 passes through exhaust lines 41temperature controlled at T7 to limit condensation of exhaust products,e.g., in the range of about 155 to 160° C. or greater as convenient, andthen into burner unit 43. The burner unit oxidizes the exhaust gases bypassing it through high temperature combustion zone 45 comprising, e.g.,H₂/O₂ combustion. The oxidized exhaust stream then passes through tube47 into countercurrent water scrubber unit 49 where it moves in acountercurrent fashion with respect to water stream 51. The water streamremoves substantially all water soluble and particulate components fromthe oxidized exhaust stream. The scrubbed exhaust gas is then releasedfrom the system 57.

The water stream with the soluble and particulate materials passes toseparator 59 where particulate components, primarily particulate galliumoxides (e.g., Ga₂O₃), are separated 61 from the water solublecomponents, primarily dissolved NH₄Cl and HCl. Separation can beobtained by known techniques, such as screening, filtering,centrifugation, flocculation, and so forth. A single embodiment of theinvention can produce 60 kg, or up to 120 kg, or more, of particulateGa₂O₃ during each month of operation. The particulate gallium oxidesgallates are collected and the Ga is advantageously recovered andrecycled into, e.g., GaCl₃ by known chemical techniques: see, e.g.,Barman, 2003, Gallium Trichloride, SYNLETT 2003, no. 15, p. 2440-2441.The water-soluble components are passed from the system.

A Preferred Particular Embodiment of the Invention

Next described is a particular preferred embodiment of the inventionthat has been generally described above. This embodiment is based on themodification and adaptation of an EPSILON® series, single-waferepitaxial reactor from ASM America, Inc. Accordingly many of thefollowing features are specific to this preferred particular embodiment.However, these features are not limiting. Other particular embodimentscan be based on modification and adaptation of other available epitaxialreactors and are within the scope of the invention.

FIGS. 2A-C illustrate aspects of GaCl₃ delivery system O₁ includingreservoir 103, which can hold 50 to 75 kg of GaCl₃ and can maintain itat as a liquid at a controlled temperature of up to about approximately130 to 150° C., and supply assembly with supply lines, valves andcontrols 105, which provide a controlled mass flow of GaCl₃ to thereactor chamber while limiting or preventing GaCl₃ condensation withinthe lines. The reservoir includes internal means for enhancingevaporation of the liquid GaCl₃. In a preferred embodiment, theseinclude a bubbler apparatus as known in the art; in alternativeembodiments, these can include means for physical agitation of the GaCl₃liquid, for spraying the liquid, for ultrasonic dispersal of the liquid,and so forth. Optionally, the supply line (or delivery line) includes acoaxial portion having an inner line conveying the carrier gas and theGroup III precursor and an enclosing coaxial line providing an annularspace inside the enclosing line but outside the inner line. The annularspace can contain a heating medium.

FIG. 2C illustrates an exemplary arrangement of delivery system 101 incabinet 135 which is positioned adjacent to conventional process gascontrol cabinet 137. To limit the length of the GaCl₃ supply line,cabinet 135 is also positioned adjacent to the reaction chamber, whichhere is hidden by cabinet 137. Process gas control cabinet 137 includes,for example, gas control panel 139 and separate portions 141-147 foradditional process gases or liquids, such as a Group III metal organiccompounds.

FIG. 2B illustrates preferred supply assembly 105 in more detail. Valves107 and 109 control lines that conduct carrier gas into reservoir 103,then through the internal bubbler in the reservoir, and then out fromthe reservoir along with evaporated GaCl₃ vapor. They can isolate thereservoir for maintenance and so forth. Valve 110 facilitates thepurging of the system above the outlet and inlet values of the containersystem. In particular, since condensation can possibly occur in thepig-tail elements 111, 112, valve 110 is useful in order to purge theseareas. Control of the container pressure in conjunction with thecontrolled temperature of the bubbler and the flow rate of the carriergas facilitates improved determination of precursor flow rate. Theaddition of valve 110 allows the complete delivery system to be purgedwith non-corrosive carrier gas when not in growth mode, thereby reducingexposure of the system to a corrosive environment and consequentlyimproving equipment lifetime. The assembly also includes valves 111-121for controlling various aspects of flow through the supply lines. Italso includes pressure controller and transducer 129 to maintain aconstant pressure over the GaCl3 container. Also provided is a mass flowcontroller 131 to provide a precise flow of carrier gas to the GaCl3container. These act to provide a controlled and calibrated mass flow ofGaCl3 into the reaction chamber. It also includes pressure regulators125 and 127. The supply line assembly, including the supply lines,valves, and controllers, is enclosed in multiple aluminum heating blocksin clamshell form to enclose each component. The aluminum blocks alsocontain temperature sensors that control supply line componenttemperatures so that the temperature increases (or at least does notdecrease) from the output of the reservoir up to the inlet of thereaction chamber. A gas heater is provided to heat the inlet gas to theGaCl3 source, preferably to a temperature of at least 110° C.

Optionally, a purifier capable of removing moisture from the carrier gasdown to no more than 5 parts per billion is placed in a carrier gasinlet line, and further a carrier gas filter is downstream of thecarrier gas purifier. The carrier gas can be optionally configured withsinusoidal bends, e.g., pigtail 112, for providing increased heatexchange surface proximate to the carrier gas heater.

FIGS. 3A and 3B illustrate top views of a preferred embodiment of thereaction chamber 201. This reaction chamber has quartz walls and isgenerally shaped as an elongated rectangular box structure with agreater width and lesser height. A number of quartz ridges 203 spantransversely across the chamber walls and support the walls especiallywhen the chamber is operated under vacuum. The reaction chamber isenclosed in a shroud that directs cooling air in order that the chamberwalls can be controlled to a temperature substantially lower than thatof the susceptor. This shroud generally has a suitcase-like arrangementthat can be opened, as it is in these figures, to expose the reactionchamber. Visible here are the longer sides 205 and the top 207 of theshroud. Susceptor 215 (not visible in this drawing) is positioned withinthe reactor. The susceptor is heated by quartz lamps which are arrangedinto two arrays of parallel lamps. Upper lamp array 209 is visible inthe top of the shroud; a lower array is hidden below the reactionchamber. Portions of the inlet manifold are visible.

FIG. 3C illustrates a longitudinal cross-section through particularpreferred reaction chamber 301 but omitting for strengthening ribs 203.Illustrated here are top quartz wall 303, bottom quartz wall 305 and onequartz side wall 307. Quartz flange 313 seals the inlet end of thereaction chamber to the inlet manifold structures, and quartz flange 309seals the outlet end of the reaction chamber to the outlet manifoldstructures. Port 315 provides for entry of processes gases, carriergases, and so forth, and port 311 provides for exit of exhaust gases.The susceptor is generally positioned in semi-circular opening 319 sothat its top surface is coplanar with the top of quartz shelf 317.Thereby a substantially smooth surface is presented to process gasesentering from the inlet manifold structures so that these gases can passacross the top of the susceptor without becoming turbulent or beingdiverted under the susceptor. Cylindrical quartz tube 321 provides for asusceptor support shaft on which the susceptor can rotate.Advantageously, carrier gas can be injected through this tube to purgethe volume under the susceptor to prevent dead zones where process gasescan accumulate. In particular, build up of GaCl₃ under the heatedsusceptor is limited.

The inlet manifold structures provide process gases through both port315 and slit-like port 329. Gases reach port 329 first though quartztube 323; this tube opens into flattened funnel 325 which allows gasesto spread transversely (transverse to process gas flow in the reactionchamber); this funnel opens into the base of the reaction chamberthrough a transversely-arranged slot in shelf 317.

With reference to FIG. 6, the funnel is compactly filled with beads ofsilicon carbide 607 and a silicon carbide insert 327 in the top of theflattened funnel provides slit-like port 329 for entry of GaCl₃ fromfunnel 325 into the reaction chamber. Two IR spot lamps 601 and theirreflector optics are located on each side of the funnel. A quartz sheath603 containing a thermocouple 605 is inserted through the bottom of thequartz tube 323 up to about the middle of the funnel height in themiddle of the SiC beads in order to enable close loop control of thespot lamp power to maintain the SiC beads at a temperature of about 800°C. Preferably, GaCl₃ is introduced through port 329 and NH₃ isintroduced through port 315. Alternatively, GaCl₃ can be introducedthrough port 315 and NH₃ can be introduced through port 329.Alternatively, an RF field may be created as known in the art in a lowerportion of tube 323 so that the NH₃ can be activated by the creation ofions or radicals. Alternatively, some or all of the NH₃ can be replacedby N₂ which will be similarly activated by the RF field. A SiC extensionplate 335 is disposed between the slit port 329 and the edge of thesusceptor. This SiC extension plate is heated by the main heating lampsto ensure that the dimer does not reform in the gas phase between theslit-like port 329 and the susceptor. The temperature of the SiCextension plate should be above 700° C. and preferably above 800° C.

FIG. 4 illustrates a diagonally cut-away view of a particular preferredreaction/transfer chamber assembly comprising wafer transfer chamber 401assembly mated to reaction chamber assembly 403. Structures which havebeen previously identified in FIGS. 2 and 3 are identified in thisfigure with the same reference numbers. Exemplary transfer chamber 401houses a robot arm, Bernoulli wand, and other means (not illustrated)for transferring substrates from the outside of the system into thereaction chamber and from the reaction chamber back to the outside.Transfer chambers of other designs can be used in this invention.

The reaction chamber assembly includes reaction chamber 301 mountedwithin shroud 405. Illustrated here are portions of bottom wall 407 andfar wall 205 of the shroud. The shroud serves to conduct cooling airover the reaction chamber to maintain a controlled wall temperature.Certain reaction chamber structures have already been describedincluding: bottom wall 305, side wall 307, flange 309 to outletmanifold, shelf 317, susceptor 215, and cylindrical tube 321 forsusceptor support and optional purge gas flow. The susceptor rotates ina circular opening 319 in rounded plate 409 which provides lateralstability to the susceptor and is coplanar with shelf 317, SiC extensionplate 335 and slit-like port 329. The planarity of these componentsensures a smooth gas flow from the gas inlet to the susceptor. Outletmanifold structures include plenum 407 which conducts exhaust gases fromthe reaction chamber in the indicated directions and into exhaust line419. The outlet manifold and flange 309 on the reaction chamber aresealed together with, e.g., a gasket or O-ring (not illustrated) madefrom temperature and chlorine resistant materials.

Inlet manifold structures (as this term is used herein) are illustratedwithin dashed box 411. Plenum 211, described below, is sealed to thefront flange of the reaction chamber with a gasket or O-ring (notillustrated) or the like made from temperature and chlorine resistantmaterials. Gate valve 413 between the transfer chamber and the reactionchamber rotates clockwise (downward) to open a passage between the twochambers, and counterclockwise (upward) to close and seal the passagebetween the two chambers. The gate valve can be sealed against faceplate 415 by means of, e.g., a gasket or O-ring. The preferred materialfor the O-ring is the same as that mentioned above for other O-rings.The gate valve preferably also provides ports for gas entry as describedbelow. The structure of the lower gas inlet, as previously described,includes communicating quartz tube 323, flattened funnel 325, andslit-like port 329.

FIG. 5 illustrates details of the particular preferred inlet manifoldstructures and their arrangement in the reaction chamber assembly.Structures which have been previously identified in FIGS. 2 and 3 areidentified in this figure with the same reference numbers. Consideringfirst the surrounding reaction chamber assembly, reaction chamberassembly 403, including shroud 405 and reaction chamber 301, is at theleft, while transfer chamber structures 401 are at the right. Susceptor215 and susceptor stabilizing plate 409 are inside the reaction chamber.Quartz flange 313 of the reaction chamber is urged against plenumstructure 211 by extension 501 of shroud 405. The reaction chamberflange and plenum structure are sealed by O-ring gasket 503 which isvisible in cross-section on both sides of port 315.

Considering now the inlet structure leading through slit-like port 329for GaCl₃ (preferably, but optionally, NH₃ instead). It is comprised ofa quartz tube 323, and funnel 325 that is longitudinally flattened butextended transversely so that it opens across a significant fraction ofthe bottom wall of the reaction chamber. Small beads or small tubes orany form of a porous IR absorbent material fill the funnel 325. Insert327 fits into the upper opening of the funnel and includes slit-likeport 329 that is angled towards the susceptor with an extension plate335 that covers the space between the susceptor and the slit-like port.In operation, GaCl₃ (and optional carrier gases) moves upward in thesupply tube, spreads transversely in the funnel, and is directed by theslit into the reaction chamber and towards the susceptor. Thereby, GaCl₃moves from port 329 towards susceptor 215 in a laminar flowsubstantially uniform across the width of the reaction chamber.

Considering now inlet structures leading through port 315, thesestructures include plenum structure 211, face plate 415, and gate valve413. NH₃ (preferably, but optionally, GaCl₃ instead) vapor is introducedinto the plenum structure through supply line 517 and passes downwardtowards the reaction chamber through the number of vertical tubes 519.NH₃ vapor then exits the vertical tubes, or optionally throughdistributed ports in which each vertical tube is lined to a group ofdistributed ports, and passes around lip 511 of the plenum. Thereby, NH₃vapor moves towards the susceptor in a laminar flow substantiallyuniform across the width of the reaction chamber. Flow through eachvertical tube is controlled by a separate valve mechanism 509 all ofwhich are externally adjustable 213. The plenum also includes tubes forconducting temperature-control fluids, e.g., GALDEN™ fluid havingtemperatures controlled so that the plenum structures through which NH₃passes are maintained within the above-described temperature ranges andso that plenum structure adjacent to O-ring 503 are maintained withinthe operational range for the sealing materials used in the O-ring. Asnoted, the preferred material for the O-ring is the same as thatmentioned above for other O-rings. Temperature control tube 505 isvisible (a corresponding tube is also visible below port 315) adjacentto O-ring 503. In typically operation, this tube serves to cool theO-ring so that it remains within its operational range.

Gate valve 413 advantageously includes a number of gas inlet ports 515as well as serving to isolate the reaction and transfer chambers. It isopened and closed to provide controlled access for wafers and substratesbetween the transfer chamber and the reaction chamber through port 315.It is illustrated in a closed position in which it is sealed to faceplate 415 by O-ring 507. In preferred embodiments, gas inlet ports 515are used to inject purge gases, e.g., N₂. Their size and spacing, whichhere is denser near the edge portions of the gate valve (and reactionchamber) and sparser at the central portions of the gate valve (andreaction chamber), are designed to improve the uniformity in compositionand velocity of the process gases as they flow across the susceptor andbuild a purge gas curtain along the side walls of the chamber to preventGaCl₃ gas from flowing underneath the susceptor to avoid undesireddeposition of GaN in this location.

Generally, for deposition of high quality epitaxial layers the inletmanifold and port structures cooperate to provide a process gas flowthat is substantially laminar (thus non-turbulent) and that issubstantially uniform in velocity and composition. The substantiallylaminar and uniform flow should extend longitudinally up to and over thesusceptor and transversely across the reaction chamber (or at leastacross the surface of the susceptor). Preferably, process gas flows inthe reaction chamber are uniform in velocity and composition across thechamber to at least 5%, or more preferably 2% or 1%. Compositionuniformity means uniformity of the III/V ratio (i.e., GaCl₃/NH₃ ratio).This is achieved by: first, designing the process gas inlet ports toprovide an already approximately uniform flow of process gases throughthe reaction chamber; and second, by designing selective injection ofcarrier gases to cause the approximately uniform flow to becomeincreasingly uniform. Control of flow downstream from the susceptor isless important.

Numerical modeling of the gas flow dynamics of the particular preferredembodiment has determined a preferred process gas inlet portconfiguration so that a substantially uniform flow is produced.Guidelines for total process gas flow rates are established according tothe selected GaN deposition conditions and rates needed for intendedsustained, high-throughput operation. Next, within these overall flowguidelines, insert 327 and slit 329 have been designed so the modeledGaCl₃ flow into the reaction chamber is substantially uniform across thereaction chamber. Also, modeling of intended GaCl₃ flows has indicatedthat after the NH₃ vapor emerges around lip 511 into the reactionchamber, this flow also becomes substantially uniform across thereaction chamber. Further, valves 509 can be controlled to amelioratenon-uniformities that may arise during operation.

Further, guided by numerical modeling, secondary carrier gas inlets havebeen added to increase the uniformity of the primary-process gasesflows. For example, in the particular preferred embodiment, it has beenfound that supply of purge gases through gate valve 413 providesimprovement by preventing accumulation of high concentrations of GaCl₃vapor between the face of gate valve 413 and lip 511 (i.e., the regionsenclosed by face plate 415). Also, it has been found that arranginginlets to provide greater carrier gas flow at the edges of the reactionchamber and lesser purge gas flow at the center also improves uniformityof composition and velocity of flow at the susceptor and bettermaintains the reactive gas above the surface of the susceptor.

Example

The invention is now compared to a standard or conventional HVPE systemto illustrate the advantages and unexpected benefits that are providedwhen conducting HVM of Group III-V material according to the invention.Prior to setting forth this comparison and by way of introduction,conventional HVPE systems are first briefly described in relevant part.

A conventional HVPE system consists of a hot-wall tube furnace usuallyfabricated of quartz. The Group III precursor is formed in-situ in thereactor by flowing HCl over a boat holding the Group III metal in aliquid form. The Group V precursor is supplied from external storage,e.g., a high pressure cylinder. Conventional HVPE has been used for thegrowth of arsenide, phosphide and nitride semiconductors. For the growthof GaN, the Group III source is typically molten Ga in a quartz boat(with which the HCl reacts to form GaCl), and the Group V source isusually ammonia gas.

In more detail, the quartz tube can be oriented either vertically orhorizontally. The surrounding furnace is usually of a resistive typewith at least two temperature zones: one for maintaining the Group IIImetal at a temperature above its melting point; and the other formaintaining the substrate/wafer at a sufficiently high temperature forepitaxial growth. The Group III-metal source equipment including a boatfor liquid Group III metal, the substrate/wafer holder, and gas inletsare placed and arranged in one end of the quartz furnace tube; the otherend serves for exhausting reaction by-products. All this equipment (orat least that which enters the furnace tube) must be fabricated ofquartz; stainless steel cannot be used. Most reactors process only onewafer at a time at atmospheric pressure. Multiple wafers must bearranged in a reactor so that the surfaces of all wafers are directly inline of the gas flow in order to achieve uniform deposition.

Wafers are loaded by first placing them on a substrate support and thenby positioning the substrate support into a high-temperature zone in thequartz furnace tube. Wafers are unloaded by removing the support fromthe furnace and then lifting the wafer off the support. The mechanismfor positioning the substrate support, e.g., a push/pull rod, must alsobe fabricated of quartz since they are also exposed to full growthtemperatures. Supported wafers, the substrate support, and thepositioning mechanism must be positioned in the usually hot reactor tubewith great care in order to prevent thermal damage, e.g., cracking ofthe wafers and/or substrate support. Also, the reactor tube itself canbe exposed to air during wafer loading and unloading.

Such conventional HVPE reactors are not capable of the sustained highvolume manufacturing that is possible with the HVM methods and systemsof this invention for a number of reasons. One reason is that thereactors of this invention require less unproductive heating and coolingtime than do conventional HVPE reactors because they can haveconsiderably lower thermal masses. In the reactors of this invention,only the susceptor (substrate/wafer support) needs to be heated, and itis heated by rapidly-acting IR lamps. Heating and cooling can thus berapid. However, in conventional HVPE reactors, the resistive furnace canrequire prolonged heating and (especially) cooling times, up to severalto tens of hours. During such prolonged heating and cooling times, thissystem is idle, and wafer production, reactor cleaning, systemmaintenance, and the like must be delayed. Furthermore, despite risks ofthermal damage, wafers are usually placed in and removed from thereactor when it is near operating temperatures to avoid further heatingand cooling delays. For these reasons, the systems and methods of thisinvention can achieve higher throughputs than can conventional HVPEsystems.

Another reason limiting the throughput of conventional HVPE systems isthat such systems require considerably more reactor cleaning that do thereactors of this invention. Because all internal components ofconventional HVPE reactors are heated by the external resistive furnace,III-V material can grow throughout the inside of the reactor, and notonly on the substrate where it is desired. Such undesired deposits mustbe frequently cleaned from the reactor or else they can form dust andflakes which contaminates wafers. Cleaning requires time during whichthe reactor is not productive.

Also, the Group III precursor is inefficiently used; most is depositedon the interior of the reactor; a small fraction is deposited on thesubstrate wafer as desired; and little or none appears in the reactorexhaust where it might be recycled for reuse. The Group V precursor isalso inefficiently used, and excess can react with unused HCl to formchlorides (e.g., NH₄Cl) that can deposit on cold areas down stream ofthe reaction zone. Such chloride deposits must also be cleaned from thereactor.

In contrast, the reactors of this invention have temperature controlledwalls so that little or no undesired growth of Group III-V materialoccurs. Reactors of this invention can be more productive sinceunproductive cleaning and maintenance either can be shorter, or need notbe as frequent, or both. For these reasons also, the systems and methodsof this invention can achieve higher throughputs that can conventionalHVPE systems.

Another reason limiting the throughput of conventional HVPE systems isthat their conventional internal Ga sources require recharging (withliquid Ga or other Group III metal) considerably more frequently than dothe external Ga sources of this invention of this invention (which arerecharged with the Ga precursor GaCl₃). The external source of thisinvention delivers a flow of Ga precursor that can be controlled in bothrate and composition at maximum sustained rates up to approximately 200gm/hr or greater. Since the capacity of the external source is notlimited by reactor geometry, it can be sufficient for many days or weeksof sustained production. For example, an external source can store up tomany tens of kilograms of Ga, e.g., approximately 60 kg, and multiplesources can be operated in series for essentially unlimited sustainedproduction.

In conventional HVPE systems, the Ga source has a strictly limitedcapacity. Since the source must fit inside the reactor and can be nolarger than the reactor itself, it is believed that an upper limit to aconventional source is less than 5 kg of Ga. For example, for 3 kg ofGa, a boat of approximately 7×7×20 cm filled with liquid Ga 4 cm deep isrequired. Disclosure of such a large Ga boat has not heretofore beenfound in the prior art. Further the rate and composition of the sourcecannot be well controlled, because the Ga precursor (GaCl) is formed insitu by passing HCl and over the liquid Ga in the Ga source boat insidethe reactor. The efficiency of this reaction is dependent upon reactorgeometry and exact process conditions, e.g., the temperature in thesource zone, and various efficiency values from 60% to over 90% havebeen reported. Furthermore, as the level of the Ga decreases and as theGa source ages, the flux of GaCl to the deposition zone can vary evenwith a constant process conditions. For these reasons also, the systemsand methods of this invention can achieve higher throughputs that canconventional HVPE systems.

Another reason limiting the throughput of conventional HVPE systems isthat heretofore their construction is not standardized, and in fact suchsystems are often individually designed and fabricated for specificusers. Lack of standardization leads to, for example, slow and complexmaintenance. Because they can often include complex and fragile quartzcomponents that are difficult to work with, such reactors aretime-consuming to disassemble and reassemble. In particular, the GroupIII source zone is intricate as it contains a separate quartz inlet forHCl, a quartz boat positioned adjacent to the HCl inlet, a separatequartz inlet for the Group V precursor (which must be kept separate fromthe Group III precursor), and a possible additional quartz inlet for acarrier gas. In contrast, the systems and methods of the presentinvention are to a great extent adaptations of tested and standardizeddesigns known for Si processing, which have been optimized for efficientoperation and maintenance and which include commercially-availablecomponents. For example, the particular preferred embodiment includes aGroup III source zone with a gate valve and Group III precursor plenumand inlet ports partially fabricated from metal. The gate valve requiresonly a short time to open and close, and the Group III precursor plenumand inlet ports are considerably less fragile. For these reasons also,the systems and methods of this invention can achieve higher throughputsthat can conventional HVPE systems.

The qualitative design choices that differentiate systems of thisinvention from conventional HVPE systems leads to surprisingquantitative benefits in epitaxial growth efficiencies, reactorutilizations and wafer production rates, and precursor utilizationefficiencies. These surprising quantitative benefits are reviewed belowusing the data in Tables 1, 2, and 3, which compare a conventional HVPEsystem designed to handle one 100 mm diameter substrate and including areactor tube of about 20 cm in diameter and about 200 cm in length witha corresponding system of this invention.

Considering first achievable epitaxial growth efficiencies, the data ofTable 1 demonstrate that the HVM systems of this invention can beconsiderably more efficient than conventional HVPE systems.

TABLE 1 Epitaxial growth efficiencies Conventional HVPE HVM Epitaxialgrowth efficiencies Reactor Information Wafer diameter cm 15 15 Reactorlength cm 200 Reactor diameter cm 20 Hot zone length cm 40 # wafersprocessed 1 1 simultaneously Reactor production times wafer load/unloadtime Pull/push rate cm/min 2 Total pull and push length cm 160 0 Totalpull and push time min 80 2 Wafer load/unload time min 9.5 2 Totalload/unload time min 89.5 3 Operation overhead % 10% 10% Totalload/unload time min 52.0 2.2 in cont. operation epitaxial growth timeTime to grow template min 0 0 Growth rate um/hr 200 (3.3) 200 (3.3)(um/min) Layer thickness um 300 300 Time to heat and cool min 0 6 Timeto grow layer min 90 90 Operation overhead % 10% 10% Total growth timemin 99 106 Total wafer-in-reactor time min 151.0 107.8 Reactorutilization (R.U.) R.U. - growth time/ % 66% 98% wafer-in-reactor timeEpitaxial growth efficiencies can be represented by the ratio of theactual epitaxial growth times to the sum of the actual epitaxial growthtimes and the reactor load/unload times. It can be seen that the HVMsystems and methods of this invention can be loaded/unloadedsignificantly faster than can conventional HVPE systems, and thus canachieve higher epitaxial growth efficiencies. It is also expected thatin actual operation, the external Ga sources of this invention willallow sustained operation for considerably longer periods than possiblewith conventional systems.

Because, in conventional HVPE system, the reactor is maintained at neardeposition temperature between runs, the substrate must be pulled fromor pushed into the reactor at a slow enough rate to avoid thermaldamage. Assuming that distance of the substrate holder from the reactorinlet is about 80 cm and a pull rate of no more that 2 cm/min to avoidthermal damage, about 40 min. are required to pull the substrate fromand also to push the substrate into the reactor. Further, once thesubstrate and wafer are positioned in the reactor, up to 10 min can berequired for thermal stabilization, reactor purge, and set-up of processgasses. (With load locks the purge and gas setup might require 5 minuteseach; without load locks, setup would be much longer.) Thus, the totalload/unload time is about 90 min, or 52 min in continuous production(where some times would be shared equally between two successive runs).

In contrast, in the HVM systems of this invention, wafers can be rapidlyloaded/unloaded at lower temperatures without risk of thermal damagethus eliminating extended wafer positioning times. Because of their lowthermal mass and IR-lamp heating, reactors used (and specifically thesusceptor and wafer in such reactors) in the HVM systems and methods ofthis invention can be rapidly cycled between higher depositiontemperatures and lower temperatures loading/unloading temperatures.Therefore, the HVM systems and methods of this invention achieveconsiderably shorter loading/unloading times than are possible inconventional HVPE reactors.

Once loaded and assuming Ga precursor sources used in conventional HVPEsystems are able to maintain an adequate mass flow rate of precursor,actual epitaxial growth times of conventional systems and of the systemsof this invention are of approximately the same magnitude. However, itis expected that the Ga precursor source used in the HVM systems andmethods of this invention has significant advantages over Ga precursorsource used in convention HVPE systems, so that in actual operation thesystems and methods of this invention will achieve relative epitaxialgrowth efficiencies even greater than the efficiencies presented inTable 1.

For example, even if capable of adequate mass flow for an initialperiod, it is unlikely that convention Ga sources can sustain adequatemass flow for extended periods. Conventional HVPE systems generate Gaprecursor in-situ to the reactor by the passing HCl gas over metallicgallium in a liquid form. Because the efficiency of this process dependsstrongly on reactor geometry and process conditions (e.g., from about60% to over about 90% depending on Ga temperature), the actual mass flowof Ga precursor (GaCl) will also vary. Further, as the level of the Gadecreases and the Ga source ages, the flux of Ga precursor can vary evenwith a constant process conditions (e.g., constant temperature and inputHCl flux). Further, conventional Ga sources (in particular the liquid Gaboat) must be within the reactor, and their capacities are thusconstrained by reactor geometry. The largest boat believed to bereasonably possible (and not believed to be disclosed in the known inthe prior art) in a conventional HVPE system could hold no more thanabout approximately 3 to 5 kg and would be approximately 7×7×20 cm insize and be filled 4 cm deep with liquid Ga.

In contrast, the HVM systems and methods of this invention employ anexternal Ga source which can provide constant, unvarying flow of Gaprecursor at up to 200 gm of Ga/hr and greater (sufficient to supportgrowth rates in excess of 300 um/hr) that can be sustained for extendedperiods of time. First, this source can provide GaCl₃ vapor in a mannerso that the Ga mass flux can be measured and controlled even duringepitaxial growth. Second, this external Ga source is capable ofsustained, uninterrupted operation because Ga precursor is supplied froma reservoir holding 10's of kilograms of precursor. Additionally,multiple reservoirs can be operated in series for effectively unlimitedoperation.

In summary, relative epitaxial growth efficiencies can be summarized byreactor utilization (R.U.) defined by the fraction of the time that awafer is in the reactor during which actual growth is occurring. It isseen that the HVM systems and methods of this invention achieve such aR.U. of about 95% or more, while conventional HVPE systems can achievesuch a R.U. of no more than about 65%. And it is expected that the HVMsystems and methods of this invention will achieve even greater relativeepitaxial growth efficiencies in actual operation.

Next considering first achievable reactor utilizations and waferproduction rates, the data of Table 2 demonstrate that the HVM systemsof this invention can be more efficient than conventional HVPE systems.

TABLE 2 Reactor utilizations and achievable wafer production ratesConventional HVPE HVM Reactor maintenance times and wafer productionrates in-situ reactor cleaning time # runs between in-situ cleaning 5 5Time to open/close reactor min 26.6 2 Total thickness to be etched um1500 300 Etch rate um/min 8 8 Etch time min 187.5 18.8 bake time min 3015 Time to load Ga with in-situ etch min 45 0.0 Operation overhead % 18%15% Total in-situ cleaning time min 339.8 41.1 ex-situ reactor cleaningtime # runs between ex-situ cleaning 15 15 time to close reactor afterunloading min 13.3 1.0 time to cool reactor min 180 20 time to takereactor apart min 120 120 time to put reactor back together min 180 120time to leak check and other min 45 45 Time to load Ga with ex-situ etchmin 10 0 time to heat reactor min 75 20 Wafer testing time min 60 60Preventive maintenance min 120 120 Operation overhead % 25% 20% Totalex-situ cleaning time min 959.2 571.2 Reactor utilization (R.U.) andwafer production rate R.U. - wafer-in-reactor time/ % 59% 76% total usetime R.U. - growth time/total use time % 39% 75% # runs (wafers) 15 15total use time for #runs (wafers) min 3734 1996 # wafers/hour 0.24 0.45# hours/wafer 4.15 2.22 # wafers/24 hours 5.8 10.8Reactors must be periodically taken out of production for cleaning andpreventive maintenance. Since the HVM systems and methods of thisinvention can be rapidly cleaned and maintained, they can achieve higherreactor utilizations and wafer production rates than can conventionalHVPE systems.

During operation, materials grow on undesired locations in the reactor,e.g., on the reactor walls and on other internal reactor components, andexcessive growth of these materials can cause problems, e.g., wafercontamination. Cleaning is required to remove these undesired materials,and can be performed either in-situ, that is without disassembling thereactor, or ex-situ, after disassembling the reactor. In-situ cleaningis often performed by etching undesired deposits with HCl. After anumber of in-situ etchings or cleanings, more thorough ex-situ cleaningis advantageous.

HVM systems of this invention require considerably less in-situ cleaningtime than conventional HVPE systems. The reactors of this invention havewalls with controlled lower temperatures so that little materialdeposits thereon during wafer production. In contrast, conventional HVPEreactors operate at higher deposition temperatures so that the sameamount of material grows on reactor walls and internal reactor parts asgrows on the wafers and substrates. Table 2 presents a scenario whichassumes that no more than 1.5 mm of unwanted GaN can be allowed todeposit on reactor walls and internal reactor parts.

For conventional HVPE systems, in-situ cleaning is required every 5runs, during which 1.5 mm of unwanted GaN (300 um per run and) will havegrown on the reactor interior. In contrast, if in-situ cleaning of thereactors of this invention is also performed every 5 runs, only anominal amount (e.g., 20% or less of the amount that will have grown inconventional HVPE systems) of GaN will have grown on the reactorinterior. (In fact, in-situ cleaning of the HVM systems of thisinvention could reasonably be delayed to only every 15 runs.) Therefore,in-situ cleaning times of conventional HVPE reactors are at least 5times (and up to 15 times) longer than the in-situ cleaning time of theHVM reactors of this invention.

Also, the HVM systems of this invention require considerably lessex-situ cleaning time than conventional HVPE systems. First, these HVMsystems have significantly shorter cooling/heating times which mustprecede and follow, respectively, ex-situ cleaning. Also, theirdisassembly/cleaning/reassembly times are similar to the shorter timesknown for Si processing systems, because the HVM systems and methods ofthis invention comprise commercially available designs and componentsalready known for Si processing. The designs and components incorporatedfrom Si processing systems include: rapidly-acting reactor gates, fullyautomated wafer handling with cassette-to-cassette loading, the abilityto perform hot load/unload, separate cooling stages, in-situ growth ratemonitoring and load locks to prevent exposure of the reactor toatmosphere.

And, as already discussed, the Ga precursor sources, i.e., the Ga boat,used in conventional HVPE systems must be periodically recharged inorder both to maintain constant precursor flow and also because of theirlimited capacity. This precursor recharging, which can be performedduring cleaning, further lengthens cleaning times of these conventionalsystems. In contrast, the external Ga sources of the HVM systems andmethods of this invention can operate with little or no interruption forextended periods of time.

In summary, reactor maintenance times can be summarized by a furtherR.U. and a wafer production rate. This second R.U. represents the ratioof the time that a wafer is in the reactor to the sum of the times thata wafer is in the reactor plus the cleaning/maintenance times. It can beseen that the HVM system and methods of this invention achieve a R.U. ofabout 75% or more, while conventional HVPE systems can achieve such aR.U. of no more than about 60%.

Relative system efficiencies can be represented by wafer productionrates, which can be derived by dividing a number of wafers produced bythe total time required to produce these wafers. Since a complete cycleof wafer production runs, in-situ cleanings, and ex-situ cleanings,rates comprises 15 runs (according to the assumptions of Tables 1 and2), these rates are determined by dividing 15 by the total time forproducing 15 wafers (including load/unload time, in-situ cleaning time,in-situ cleaning time, maintenance time, and source recharge time). Itcan be seen that the total time the HVM systems and methods of thisinvention require to produce 15 wafers (runs) is considerably shorterthan the total time required by convention HVPE systems. Therefore, thesystems and methods of this invention achieve an approximately 2 foldthroughput improvement over the prior art. As discussed above, a greaterthroughput improvement is expected during actual operation.

Lastly, considering comparative precursor efficiencies, the HVM systemsand methods of this invention utilize precursors, especially Gaprecursors, more efficiently than conventional HVPE systems. This isexemplified by the data in Table 3.

TABLE 3 Precursor utilizations Conventional HVPE HVM Precursorutilization ammonia (both processes) Ammonia Flow slpm 14 10 Totalammonia flow time min 132.0 97.7 Total ammonia for 90 min. run mole 82.543.6 HCl (convention HVPE) Moles of HCl/min during run mole/min 0.024Liters HCl used in run liter 51.2 gallium (convention HVPE) Input V/IIIratio 30 Moles/min of ammonia during mole/min 0.6250 run Moles/min of Garequired by mole/min 0.0208 ammonia flow Conversion of GaClx to GaN %95% Actual moles/min of Ga used mole/min 0.0219 in run Additional molesof Ga % 10% moles of Ga/min for run mole/min 0.024 Weight of Ga/min forrun gm/min  1.76 gm Ga/min; 1000 gm Ga/hr Weight of Ga per run gm 151.4gallium (HVM) Input V/III ratio 30 moles of ammonia/min during mole/min0.4464 run moles/min of Ga to meet V/III mole/min 0.0149 Conversion ofGaClx to GaN % 95% moles of GaCl3 dimer/min mole/min 0.0082 required tomeet V/III Additional moles of GaCl3 dimer % 10% Total moles GaCl3 dimerfor run mole 0.82 Atomic weight GaCl3 dimer gm/mole 352.2 Total weightof GaCl3 dimer gm 287.4 for run Percent of GaCl3 dimer that % 40% is GaWeight of Ga for run gm 114 Weight of Ga/hour for run gm 75 gm Ga/hr Gautilization % 21% 25% Utilization with Ga recycling % 27% 80% (est.)

Ga utilization is determined in Table 3 by, first, considering that aconventional HVPE system suitable for a 15 cm wafer can be expected touse approximately 14 slpm (standard liters per minute) of ammonia.Assuming a V/III ratio of 30 and a 95% conversion of the Ga precursorinto GaN, the conventional system can be expected to use approximately1.8 gm/min of Ga. A 90 minute run sufficient to grow 300 um of GaN at200 um/hr therefore requires about 151 gm of Ga. Since there is about 31gm of Ga in a 300 um layer on a 15 cm wafer, the Ga efficiency of theconventional HVPE reactor is approximately 21% (= 31/151). Since most ofthe remaining 120 gm (=151-31) is deposited on the insides of thereactor, little is thus unavailable recycling and reuse. It is expectedthat even with recycling and reuse of Ga exhausted from the reactor, theGa efficiency of the conventional HVPE reactor is no more thanapproximately 25%.

In contrast, HVM systems and methods can be expected to use a lowerammonia flow (e.g., 10 slpm) and therefore a lower Ga flow and a lowertotal Ga required for a 15 cm wafer (e.g., 114 gm). Therefore, the HVMsystems and methods of this invention can achieve Ga efficiencies of 27%(= 31/114) without recycling and reuse and up to perhaps 80% or greaterGa efficiency with recycling and reuse of Ga exhausted from the reactor.Additionally, since little of the remaining 83 gm (=114-31) is depositedon the insides of the reactor, most of this unused Ga appears in thereactor exhaust where it is available recycling and reuse. It isexpected that with recycling and reuse of exhaust Ga, the Ga efficiencyof the HVM systems and methods of this invention can reach 80% orgreater.

The preferred embodiments of the invention described above do not limitthe scope of the invention, since these embodiments are illustrations ofseveral preferred aspects of the invention. Any equivalent embodimentsare intended to be within the scope of this invention. Indeed, variousmodifications of the invention in addition to those shown and describedherein, such as alternate useful combinations of the elements described,will become apparent to those skilled in the art from the subsequentdescription. Such modifications are also intended to fall within thescope of the appended claims. In the following (and in the applicationas a whole), headings and legends are used for clarity and convenienceonly.

A number of references are cited herein, the entire disclosures of whichare incorporated herein, in their entirety, by reference for allpurposes. Further, none of the cited references, regardless of howcharacterized above, is admitted as prior art to the invention of thesubject matter claimed herein.

1. A system for providing high volume production of a monocrystallineGroup III-V semiconductor material, which comprises: an external, highvolume source of a Group III precursor for use as one reactant, anexternal, high volume source of Group V component for use as anotherreactant, and a temperature controlled reaction chamber that receivesthe Group III precursor and Group V component for reaction to form themonocrystalline Group III-V semiconductor material.
 2. The system ofclaim 1 wherein the high volume source of Group III precursor comprisesa container for holding the precursor, a heating arrangement for heatingthe precursor and generating a gas flow of the precursor, and a massflow controller for introducing a desired amount of gaseous precursorfor reaction with the Group V component to form the monocrystallinesemiconductor material.
 3. The system of claim 2 wherein the containeris operatively associated with a source of carrier gas and a relatedconduit that introduces the carrier gas into the container in a mannerwhich facilitates formation of the gas flow of the precursor.
 4. Thesystem of claim 1, wherein the reaction chamber has exteriorly cooledwalls to prevent or minimize deposition of reactants thereon.
 5. Thesystem of claim 1 which further comprises a rotatable support forholding one or more substrates upon which the monocrystallinesemiconductor material is to be deposited, and the reaction chamberincludes entry apertures for introducing the reactants into the reactionchamber in a controlled manner with the substrates and supportconfigured, dimensioned and positioned with respect to the reactantentry apertures to facilitate formation and deposition of themonocrystalline semiconductor material upon the substrate(s).
 6. Thesystem of claim 5 wherein the reaction chamber is configured as ahorizontal, rectangular chamber and the apertures for introducing thereactants are located in different portions of the chamber to avoidreaction until they meet at a predetermined location adjacent andimmediately above the substrate(s).
 7. The system of claim 6 wherein thereaction chamber includes a floor, a ceiling, a pair of sidewalls, anopen inlet, an open outlet, and one reactant entry aperture comprises ahorizontal slot in the floor for introducing that reactant into thereaction chamber, with the slot configured and dimensioned to introducethe reactant and to direct it to the predetermined location for reactionwith another reactant.
 8. The system of claim 7 which further comprisesa reactant introduction device that includes a nozzle containing heattransfer materials therein wherein the device is operatively associatedwith a heating source to heat the reactant prior to its introductioninto the reaction chamber.
 9. The system of claim 7 wherein the entryaperture for the other reactant comprises an injection nozzle and thesystem further comprises a passage for introducing one or more purge gasstreams into the reaction chamber to direct the other reactant to thepredetermined location to efficiently form the monocrystalline materialon the substrate(s).
 10. The system of claim 9 which further comprises agate valve associated with the inlet for introducing the purge gasstreams in a manner that directs the other reactant to the predeterminedlocation to efficiently form the monocrystalline material upon thesubstrate(s).
 11. The system of claim 10 wherein the reaction chamber ismade of quartz, the gate valve is made of metal, and which furthercomprises an O-ring located between the gate valve and reactor forproviding a sealing connection, wherein the gate valve is cooled by thepurge gas to avoid deteriorating the O-ring during operation.
 12. Thesystem of claim 10 wherein the gate valve includes a plurality of spacedapertures to assist in directing the other reactant to the predeterminedlocation to form the monocrystalline material.
 13. The system of claim 1wherein the reaction chamber includes one or more walls and issurrounded by an enclosure which is operatively associated with one ormore fans for circulating air in the enclosure to lower the temperatureof the reaction chamber wall(s) to reduce or prevent deposition of theGroup III precursor or reaction byproducts on the reaction chamberwall(s) to provide a longer operating time before maintenance isrequired.
 14. The system of claim 13 wherein the reaction chamberincludes an outlet and exhaust conduit for removing exhaust gases,including unreacted Group III precursor, unreacted Group V component andreaction byproducts, from the reaction chamber, and a heatingarrangement for heating the outlet and exhaust conduit to a temperaturesufficient to prevent condensation of the exhaust gases therein.
 15. Thesystem of claim 14 which further comprises a pump for removing theexhaust gases from the reaction chamber, a thermal destruction chamberfor thermally destructing the exhaust gases to form Group III oxidesolids, a scrubber for washing the destructed gases, and a filter forrecovering the Group III oxide solids for recycling and formation ofadditional Group III precursor.
 16. The system of claim 1 which furthercomprises a recovery system for treating unreacted Group III precursorto recover its Group III element for recycling and formation ofadditional Group III precursor.